Optical disk media, such as a DVD-RAM and a DVD-RW, that use a rewritable type phase change optical recording material, have been available widely for general use. Recently, a larger capacity Blu-ray Disc, that deals with the video recording of high-definition television broadcasting, has been manufactured as a commercial product using a blue laser. In the future, the realization of an optical disk apparatus that enables recording/reproduction on both a DVD and a Blu-ray Disc by a drive is anticipated. The optimization of the power and pulse conditions of a recording laser beam, called “trial writing”, is important in recording information in these high-density optical disks.
Usually, a jitter, that is a standard deviation of the shift between a data edge and a clock edge, is used to estimate the quality of a signal recorded in an optical disk. A measuring apparatus, such as an exclusive jitter analyzer, is necessary to measure the jitter. Because an expensive jitter analyzer cannot be incorporated in an optical disk apparatus, a signal estimation index was required instead of this expensive jitter analyzer.
As an example of a conventional trial writing technique, the technique of estimating jitter equivalently and optimizing the recording power using this value by generating a logical pulse, that is called an error pulse, and counting an error pulse count, instead of measuring the jitter directly, when the phase difference between the data edge and the clock edge exceeds a predetermined value, is disclosed in JP-A No. 320777/1998.
Moreover, a trial writing technique for a 4.7 GB DVD-RAN that uses an adaptive recording strategy of a table reference type conforming to the front and rear space length and mark length is disclosed in the International Laid-Open Patent Publication No. WO01/011614. In accordance with this technique, classification processing is applied by making an error pulse correspond to a table of the recording strategy, and a pulse condition of a recording laser beam is optimized so that the error pulse values of table items may be minimized. Practical classification processing requires two 4×4 tables. However, because a logical pulse, called an error pulse, is used to estimate a signal, the classification processing can be realized simply by a logic LSI.
[Patent document 1] JP-A No. 320777/1998
[Patent document 2] WO01/011614
As described previously, at the present time, when the use of a DVD has become widespread, a fast recording/reproduction speed is becoming a most important technical development problem. Because the noise effect increases when a signal is reproduced at high speed, a PRML (Partial Response Maximum Likelihood) method that improves the S/N (Signal-to-Noise) ratio effectively is becoming indispensable as a binary system of a reproducing signal instead of a conventional direct slice method. The PRML method requires a process of digitalizing the reproducing signal at every clock using an AID (Analog-to-Digital) converter. Necessarily, a PLL (Phase-Locked Loop) circuit also changes from a conventional analog system to a digital system. In the PLL circuit of the digital system, a VCO (Voltage Controlled Oscillator) circuit is operated so that the level of the reproducing signal at an edge point may approach zero, instead of directly comparing the phase difference between a data edge and a clock edge. Because a conventional error pulse generation circuit adapts to a phase error detection circuit of the PLL of an analog system, it could not be adapted to the PLL of a digital system.
When a PRML method and a digital PLL are adopted in this manner, a conventional error pulse generation circuit will not function. When a new signal estimation index is introduced instead of an error pulse, the configuration of a conventionally developed logic LSI, and a control program resource will not be able to be used effectively.
When a detection window clock period (PLL clock period) is Tw, the length of the shortest signal recorded in a DVD is 3 Tw. In a DVD, the jitter of a repetition signal having the shortest signal length is about 6%, and is satisfactory equally with the jitter of a random signal so that ample reliability may be obtained by reproducing the signal using a conventional direct slice method. On the other hand, in a Blu-ray Disc, the length of the shortest signal recorded to improve code efficiency is 2 Tw. The jitter of the repetition signal having the shortest signal length is about 10%, and the quality is bad in comparison with the jitter of the random signal. Accordingly, in the methods disclosed in the above-referenced JP-A No. 320777/1998 and WO01/011614, many error pulses are generated from the edge of a 2 Tw mark, and error pulse is generated from the edge of a 3 Tw or more mark. Accordingly, in order to obtain a recording condition of the 2 Tw mark, because the minimum value of an error pulse count is hard to detect, and the error pulse count is kept at 0 even if the recording condition of the 3 Tw or more mark is changed, the recording condition can not be obtained accurately.